The invention relates to a device for measuring accelerations for a vehicle passenger protection system.
In order to improve road traffic safety, improvements are continuously being made to already known passenger protection systems in vehicles, in order to provide the best possible protection for vehicle passengers in cases where the vehicle collides with an obstacle or another vehicle, for example. Typically, a passenger protection system will comprise a control device with a sensor arrangement, which may comprise one or more acceleration sensors, and an evaluation unit, one or more satellite sensors which can also comprise one or more acceleration sensors, and suitable protective devices such as airbags, belt tighteners and roll bars.
In order to measure, obtain or record (these terms will be used below synonymously) the accelerations which are generated by a collision, different systems with different arrangements of acceleration sensors are known. A differentiation is made between sensors which are arranged centrally in the vehicle, and additional sensors—“satellite sensors”—which are arranged decentrally in the vehicle. The latter are usually arranged near the outer shell of the vehicle, the precise location being selected in accordance with the specific function of the sensor.
The sensors which are sensitive to accelerations are referred to synonymously below as “acceleration sensors”, “acceleration recording devices” or, in an even more abbreviated form, as “sensors”. These generally comprise an directional characteristic with regard to the direction of the influential acceleration in relation to the alignment of the sensor. The directional characteristic specifies a main sensitivity direction, in which the measuring sensitivity of the sensor, i.e. the strength of its output signal in relation to the value of the acceleration, is at maximum level when the direction of the acceleration is aligned in parallel or antiparallel with the main sensitivity direction.
In the following, the assumption is made that all the directions mentioned above are located approximately at the same level, in particular, at the level in which the vehicle is moving. Furthermore, it is assumed that each sensor is electrically connected with the relevant evaluation channel, whereby the measuring signal directly generated by the sensor is processed and/or is evaluated at least partially, and the evaluation unit is fed into the control device. An evaluation channel may comprise an analogue-digital converter, a switch to filtering (for example equalisation), at least one switch to integration, a switch to the comparison of the signal with a prespecified threshold value and/or logical switches to linking signals which have been generated in the same sensor or by another sensor, or by another means. In the simplest case, an evaluation channel solely comprises devices to forward the measuring signal generated by the sensor to the evaluation unit. In the evaluation unit, the signals from the evaluation channels are processed and/or evaluated, and a trigger signal is generated in order to trigger the safety facilities in the vehicle.
In the evaluation channels, and, when appropriate, in the central evaluation unit, analogue measuring signals from the sensors (such as electrical voltages) and the analogue signals generated during the processing stages can be interpreted in relation to reference values (such as a reference voltage). The reference values can be different for different evaluation channels, or can essentially be the same.
In simple processing cases, the signal generated by a sensor which corresponds to an acceleration, for example, is equalised in the evaluation channel in a low-pass filter, and if appropriate, is integrated in an integrator, producing a signal which corresponds to an influential speed. The acceleration and/or speed signal is compared to a threshold value. If the signal exceeds the threshold value, a trigger signal is produced. In order to avoid the safety facility from being triggered unintentionally, for example due to a measuring error or another malfunction of the sensor or another switching element in the evaluation channel, a trigger signal of this type can be subjected to at least one verification check, in which the trigger signal is linked to at least one verification signal which is generated by an independent evaluation. In simple cases, the trigger signal can be linked, for example, to a trigger signal which is generated independently in a different evaluation channel using logical “AND” switching.
In order to cover in the best possible way all the directions of the accelerations at the level of movement which affect the vehicle, and in order to determine the value and direction of the influential acceleration, two acceleration sensors are usually provided (which are preferably positioned at right-angles to each other). With a single acceleration sensor, however, only those accelerations can detected which are travelling in a single direction. When the acceleration sensor is directed towards the longitudinal vehicle axis, for example, it is not able to detect a collision from the side, and when there is a collision with an acceleration component in the direction of the longitudinal vehicle axis, a differentiation can be made, using the preceding sign of the measured acceleration components, as to whether the components are aligned from front to back (as is the case with a frontal collision) or vice-versa (as is the case with a rear collision).
In order to provide two “full coverage” acceleration sensors, two preferred sensor arrangements have been established in practise:                a) the two sensors are aligned along the longitudinal vehicle axis (X installation axis) and a transverse vehicle axis (Y installation axis), or        b) both sensors are essentially aligned in the direction of the longitudinal vehicle axis at an angle of ±45 degrees.        
Arrangement a) will not be discussed here in further detail. Arrangement b) is explained below in reference to FIG. 1, since this arrangement can be used as a basis for explaining in a simple manner the effect and the advantage of the invention. This arrangement, however, is only used as an example of arrangements with sensors which are generally aligned at an angle to the longitudinal or transverse axis of the vehicle.
EP 0 311 039 A2 discloses a sensor arrangement with three acceleration recorders, of which the three sensitivity directions are in each case arranged offset from each other by 120° in order to measure acceleration components from all directions. DE 38 16 589 A1 and DE 100 02 471 A1 both disclose a sensor arrangement with two sensors, of which the sensitivity directions of the first sensor form an angle of 45 degrees, and those of the second sensor form an angle of 135 degrees to the longitudinal vehicle axis. However, none of these publications deal with the problem of the influence of the change in a reference value on the tripping behaviour of a passenger protection facility.